Tools for oil and gas wellbore made from tubular composites have a need for a high strength thread form. Specifically, high strength threads are needed for tooling connections and for electrical isolation.
In general composite construction, there are limited methods for assembling multiple components. The primary method is to bond parts together using an adhesive. In applications requiring greater strength, mechanical reinforcement may be provided using dowel pins or bolts. In the special case of tubular composites, threading may be used. However, typical thread forms used on tubular composites are generally not well suited for high load applications. The construction of the tubular composite is a limiting factor in the connector strength.
A tubular composite is made up of a reinforcing material and a resin. The reinforcing material can be any of a large number of materials commonly used in composite manufacture. Glass is a common reinforcement and is available in woven cloths, filaments (yarn), and tape forms. Additionally, woven cloths, filaments, and tape forms each have a large number of configurations available. For oil and gas wellbore applications, epoxy resins are commonly used for the elevated temperature and pressure requirements. Generally, epoxy resins have good chemical resistance required in this application.
FIG. 1A illustrates the construction of tubular composite as commonly used in oil and gas wellbore tools. The tubular composite may be constructed by wrapping a reinforcing cloth 10 around a tooling mandrel 9 with the addition of a resin 14 either during (wet wrapping) or after (resin transfer molding) wrapping. Alternatively as shown in FIG. 1B, a tubular composite may be constructed by the process of filament/tape winding, wherein reinforcing filaments 11 are wound around the tooling mandrel 9 with the addition of the resin 14. The latter method is commonly used by the manufacturers of fiberglass tubing.
Once combined in a tubular form, the material is subjected to heat to harden the resin system. This hardening is known as “curing”. Curing requirements are determined by the resin used. The composite material may be post-cured for improved properties.
Material produced in this manner is referred to as a laminate 12,13,14 within the industry. A laminate has layers of reinforcing material 12 and 13 with a resin bonding layer 14 between them as shown in FIG. 1C and FIG. 1D [check Section 1-1]. The area between any two layers is referred to as an interlaminar area.
The material properties of laminates 12,13 are typically anisotropic. An anisotropic material is one with properties that vary based upon the load orientation. Referring to FIGS. 1C and 1D, the material properties in tubular composites are oriented in radial 15, circumferential 16, and axial 17 directions. Metals are classified as an isotropic material. An isotropic material is one where the material properties are the same regardless of the load orientation. Common thread forms have been designed based upon the assumption that the materials used are isotropic.
In the prior art, the thread forms were not modified to account for an anisotropic material properties when manufactured in laminate.
As the properties of laminates differ in each direction, the load condition will determine the performance. In FIG. 1D, when a load is applied to the laminate 12,13,14 in an axial direction 17 such that two adjacent layers 12,13 are subject to opposing loads 18,19 where the load 19 condition is termed an interlaminar shear. The interlaminar shear occurs in the resin 14 that bonds the two layers together. This strength of the material in this interlaminar area is significantly less than the strength in other directions. In a tubular composite, the interlaminar shear condition occurs in a direction 17 along the axis of the tube formed by the cross-section 1-1 of FIG. 1. A thread form machined into such a tubular composite will be subjected to this interlaminar shear.
In the manufacture of metallic components for oil and gas wellbore applications, there are many thread types available based upon the application. Thread manufacture in tubular composites has borrowed thread forms from the metallic threads. For oil and gas wellbore, a thread form that is used for tubular materials is the American Petroleum Institute (API) External Upset 8 Round Thread (API EUE 8RD). The details of this thread may be found in the API specification 5B, “Specification for Threading, Gauging and Thread Inspection of Casing, Tubing, and Line Pipe Threads”. The API EUE 8RD has primarily been borrowed for use in fiberglass tubing, a tubular composite material with thin wall sections. FIG. 2 illustrates a typical Box 20 and Pin 21 configuration for the API EUE 8RD thread as used in common tubular both metallic and non-metallic. The API EUE 8RD thread form allows for a moderate pressure seal, but was not intended to carry the high loads that are required in the tooling applications for which the new thread is intended.
For oil and gas wellbore tooling applications, there are other threads that offer greater strength. Among such connections, the most common threads used are the American National Standard Acme Screw Threads (Acme) as specified in ANSI B1.5-1988 (R2001), shown in FIG. 3. If a shallower thread is required, ANSI B1.5-1988 (R2001) specifies a related thread form known as the American National Standard Stub Acme Screw Threads (Stub Acme). The Acme and Stub Acme thread share a common thread angle 33 (for second thread angle where in FIG. 3), but vary in thread height 36. All dimensions for these thread can be determined for both the internal (box) 30 and external (pin) 31 threads based upon the major diameter 34 of the external thread and the thread pitch 35. As with other thread forms, the Acme and Stub Acme thread forms were designed for metallic tubular.
A thread form used in some applications where high strength is required in metallic-tubular connections is the American National Standard Inch Buttress Screw Thread (Buttress) as specified in ANSI B1.9-1973 (R1992). As indicated in FIG. 4, the Buttress thread has a load bearing flank 43 that is nearly perpendicular to the tubular axis 4-4. As with the Acme and Stub Acme threads, most dimensions may be calculated from the major diameter 46 of the pin thread 41 and the thread pitch 42.
All of the threads above have been designed for metallic connections. The isotropic properties of metals allow for such variation in thread designs. When applied to tubular composites, the anisotropic properties yield poor performance. The common failure for threads manufactured in tubular composites is a shear failure of the threads. The shear failure of the threads is an interlaminar shear which typically occurs at the root of the thread. The interlaminar shear strength of the tubular composite is significantly weaker than the strength in other directions. To carry the high loads required in many oil and gas wellbore applications, it becomes necessary to have a thread engagement that is far in excess of what is easily manufacturable.